In recent years, various attempts have been made to improve purification performance of a honeycomb structure used as an exhaust gas purification catalyst body, filter, adsorbent, or the like in order to deal with tightened exhaust gas regulations.
For example, attempts have been made to enable a honeycomb structure used as a catalyst body to exhibit a desired exhaust gas purification performance immediately after starting an engine by rapidly increasing the catalyst temperature to increase catalytic activity by reducing the thickness of the partition wall to reduce heat capacity. Attempts have also been extensively made to increase the porosity of the partition wall of a honeycomb structure used as a filter or the like in order to increase collection efficiency or the like.
The above honeycomb structure is held on the outer circumference surface and disposed in a casing. However, since the honeycomb structure is placed at a position, near an engine, at which the honeycomb structure is continuously subjected to a large amount of vibration, the honeycomb structure must be securely held so that it is not removed due to vibration (clamshell, stuffing, tourniquet, swaging, or the like is used as the holding method (canning method)).
The honeycomb structure is used under high-temperature environment in which high-temperature exhaust gas is blown against the honeycomb structure. In the case of using the honeycomb structure as a filter, the honeycomb structure is subjected to a high-temperature regeneration treatment when a predetermined amount of soot has been deposited. Therefore, it is also important that the honeycomb structure have high isostatic strength and thermal shock resistance.
However, the above-mentioned progress of a decrease in thickness and an increase in porosity causes a decrease in isostatic strength and thermal shock resistance of the honeycomb structure, thereby causing damage to the partition wall as another problem. Therefore, a honeycomb structure having high isostatic strength and thermal shock resistance while satisfying a demand for an increase in purification performance has been demanded.
In view of the above-described situation, JP61-47135B, JP60-78707A, JP62-114633A, and JP10-180915A disclose a “honeycomb structure and the like including cells having a rectangular cross-sectional shape” as a conventional honeycomb structure.
However, since the cells of the honeycomb structure are provided at a uniform cell density in order to improve the thermal shock resistance or the heat exchange ratio, these honeycomb structures do not exhibit sufficient isostatic strength under practical environment in which vibration is continuously applied. Therefore, these honeycomb structures cannot withstand long-term use.
JP57-110314A, JP-U59-47310A, and JP-A-55-155742 disclose a honeycomb structure and the like including a low-cell-density region and a high-cell-density region.
However, since these honeycomb structures and the like merely aim at making the exhaust gas flow rate uniform, sufficient characteristics cannot be obtained for thermal shock resistance.
Specifically, since the partition walls of the high-cell-density region and the low-cell-density region are intricately bonded in these honeycomb structures and the like at the boundary between the high-cell-density region and the low-cell-density region, stress caused by thermal shock tends to be concentrated at such a boundary of the honeycomb structure. Therefore, these honeycomb structures and the like exhibit insufficient thermal shock resistance under high-temperature environment such as during the regeneration treatment when used as a filter due to damage to the partition wall or the like. Moreover, since it is very difficult to manufacture a die corresponding to the complex shape at the boundary, these honeycomb structures and the like are rarely used in practical application.
JP-U60-145216A discloses a “catalytic converter in which cells having a trapezoidal cross-sectional shape are provided in a state in which the open area of each cell is increased stepwise from the center section toward the outer circumference”, JP-U62-1 63697A discloses a “honeycomb structure in which triangular cells formed by providing a partition wall which connects opposite angles of a square cell are provided near the outer circumference of the honeycomb structure”, and U.S. Pat. No. 3,853,485 discloses a “honeycomb structure in which the number of partition walls provided between two opposite sides of an octagonal cell is increased stepwise from the outer circumferential section toward the center section of the honeycomb structure”.
However, since the partition walls of these honeycomb structures have a complicated bent structure, it is very difficult to manufacture a die for forming these honeycomb structures, whereby cost of the resulting honeycomb structure is increased to a large extent. Therefore, these honeycomb structures are rarely used.
JP-U60-147711A discloses a “honeycomb structure in which all cells have a tetragonal cross-sectional shape and the sizes of the cells are gradually increased from the center section toward the outside”, and JP9-158720A discloses a “honeycomb structure in which a plurality of flow straightening plates are disposed in the shape of a lattice so that the intervals between the plates are decreased from the center section to the peripheral section of the casing”.
However, since the sizes of the cells are varied at a high ratio over the entire region in order to control the exhaust flow rate distribution or the like, the flattening ratio (long side length/short side length) of the cell is increased. Moreover, the ratio of the cell side length to the partition wall thickness is not taken into consideration. Therefore, a honeycomb structure having thin partition walls cannot be properly subjected to canning due to extremely low isostatic strength.
As a die used to manufacture a honeycomb structure in which the partition wall intervals are varied, a die in which holes having the same inner diameter and the same material path length are provided at equal intervals at intersections has been used in the same manner as in the case of manufacturing a honeycomb structure in which the partition walls are provided at equal intervals (JP 60-78707A). However, in a die used to manufacture a honeycomb structure in which the partition wall intervals are varied, open slit areas per unit cell pitch also differ corresponding to the change in the partition wall interval. Therefore, if holes having the same inner diameter and the same material path length are provided to each cell pitch, the raw material extrusion speed is increased in the area in which the slit interval is small and the raw material extrusion speed is decreased in the area in which the slit interval is large, whereby a forming failure of the resulting honeycomb structure tends to occur.